1. Field of the Invention
The present invention relates to a thermal head, particularly relates to a thermal head wherein a conductive layer electrically connected to a common electrode is formed on a heat insulating layer and a heater element is laminated on the conductive layer via a layer insulating layer.
2. Description of the Prior Art
Heretofore, a general thermal head as a recording head mounted in a thermal printer and others records on thermosensitive recording paper in colors by arraying plural heater elements composed of a heating resistor in one or plural columns on a substrate and selectively energizing and heating each heater element according to record information or prints on various record media by melting ink of an ink ribbon and transferring it on a record medium such as plain paper and paper for an overhead projector (OHP).
Such a conventional type thermal head is provided with a substrate 124 made of a ceramic or silicon and others and having an insulating layer as shown in FIGS. 24 and 25, and a convex portion 124a the section of which is approximately trapezoidal is formed in the vicinity of the right end of the upper surface of the substrate 124 shown in FIG. 25.
Also, a heat insulating layer 125 is formed on the upper surface of the substrate 124 and plural heater elements 126 formed in a desired pattern by executing plasma etching by gas acquired by mixing fluoric gas and gaseous oxygen after a heating resistor made of cermet such as Ta--SiO.sub.2 is formed by sputtering and others and hereby, arrayed in a row according to the number of dots corresponding to desired resolution are provided on the upper surface of the heat insulating layer 125 on the convex portion 124a.
Further, a common electrode 127 is formed on the right side of each heater element 126 shown in FIG. 25 and is formed in a U shape along the outside margin of a thermal head 123 as shown in FIG. 24.
A common electrode terminal 127a is formed at two ends of the right and left outside margins of the common electrode 127. A terminal plating layer 127b is applied to the upper surface of the common electrode terminal 127a.
In the meantime, plural separate electrodes 128 for independently energizing each heater element 126 are patterned above the left side of the heater element 126 shown in FIG. 25. A connecting electrode 129 on the separate electrode side for electrically connecting the separate electrode 128 and the heater element 126 is formed between the separate electrode 128 and the heater element 126. The left end of the separate electrode 128 shown in FIG. 25 is equivalent to a separate electrode terminal 128a and a terminal plating layer 128b is applied to the upper surface of the separate electrode terminal 128a.
The common electrode 127 and the separate electrode 128 are made of Al or Cu and others, are formed in a predetermined pattern by etching and others after Al or Cu is deposited on the upper surface of the heater element 126 and the heat insulating layer 125 by sputtering and others until Al or Cu is approximately 2 .mu.m thick, and a part between the common electrode 127 and the separate electrode 128 of each heater element 126 is equivalent to a heating part 126a.
Also, a passivation layer 130 for preventing oxidation and abrasion is formed on/over the upper surface of the heat insulating layer 125, the heater element 126, the common electrode 127 and the separate electrode 128. The passivation layer 130 is made of SIALON (Si--Al--O--N compound solid solution) excellent in oxidation resistance and abrasion resistance and is laminated by sputtering so that any upper surface except the separate electrode terminal 128a and the common electrode terminal 127a is coated.
In the conventional type thermal head 123 having configuration described above, predetermined recording is performed by selectively heating the heater element 126 by selectively energizing between the common electrode 127 and the separate electrode 128, melting ink of an ink ribbon touched to the part and transferring it on recording paper or directly coloring thermosensitive recording paper.
Also, in the conventional type thermal head 123, as described above, the convex heat insulating layer 125 is provided near to the end of the substrate 124 with the layer biased, the heating part 126a is formed on the convex heat insulating layer 125 and the quality of recording is enhanced by using such a thermal head 123.
However, the conventional type thermal head 123 has a problem that as the common electrode 127 is extremely elongated in a narrow part between the end of the substrate 124 and the heater element 126 because the respective heater elements 126 are formed near the end of the substrate 124 as shown in FIG. 24, the resistance value of the electrode is increased even if the common electrode 127 is made of copper or aluminum, so-called voltage drop occurs in the common electrode 127 and dispersion occurs in exoergic temperature among the heater elements 126.
Further, the length of an energizing path from the heater element 126 to the common electrode terminal 127a differs every heater element 126 and the heater element 126 near the center of the row is the farthest from the common electrode terminal 127a. Therefore, the quantity of voltage drop in the common electrode 127 connected to the heater element 126 near the center of the row is increased and the exoergic temperature of the heater element 126 near the center of the row sometimes lowers.
In thermal imprint printing for example when voltage drop occurs in such a thermal head and the exoergic temperature of the heater element 126 lowers, there is a problem that the quality of printing is deteriorated such as the unevenness of density in printing is increased.
For a countermeasure against voltage drop, it is conceivable to reduce the quantity of voltage drop by thickening the thickness of the common electrode 127, however, when the thickness of the common electrode 127 is thickened, it becomes difficult to laminate the passivation layer 130 on the common electrode 127, in addition, in printing, the pressure of the thermal head 123 and a platen of a printer not shown concentrates at the heating part 126a and the common electrode 127 adjacent to the heating part 126a and large shearing stress is applied to the common electrode 127.
Therefore, there is a defect that material fatigue (metal fatigue) occurs in the common electrode 127 made of soft metallic material such as Al and Cu, the passivation layer 130 is peeled or destroyed early, disconnection due to abrasion and corrosion occurs in the common electrode 127 and the life is short.
Also, voltage drop is inhibited and the difference of exoergic temperature in the position of each heater element 126 can be removed by using a well-known thermal head called an alternate lead head adjacent to the row of each heater element 126 and provided with plural adjacent common electrodes, however, in this case, there is also a defect that as the adjacent common electrode is formed between adjacent heater elements, a dimension between heater elements is increased, a dimension between dots of a printed image is increased and high definition printing is difficult.